U.N. spokesman Eduardo Del Buey said from U.N. headquarters the U.N. Mission in South Sudan confirmed that armed gangs clashed with the Sudan People's Liberation Army in the eastern town of Pibor in the state of Jonglei.
SPLA is the South Sudanese military. UNMISS said it was providing shelter for more than 2,000 people who fled weekend violence in the region.
A U.N. report in June said violence between rival ethnic groups in Jonglei led to deaths, abductions of women and children, destruction of homes and the displacement of thousands of people.
The report described a "cycle of violence" in the region, near South Sudan's border with Ethiopia.
Del Buey said UNMISS authorities had reached out to the SPLA and South Sudanese security officials in an effort to find a way to defuse the situation.
"UNMISS remains committed to protecting civilians but underscores that the government of South Sudan is primarily responsible for the maintenance of security and the protection of civilians," he said in a statement.